msh2vtu overhaul

msh2vtu is an important part of OGSTools, but was in a hard to maintain state, thus it got completely overhauled and cleaned up. This succeeded in making a shorter, more comprehensible version.

• argument -k was removed (keep_ids): in our OGS world, there is no reason to keep the gmsh data names and the wrong data types in the meshes, which would happen if k was used. Thus, I see no reason to keep this.
• argument log_level was simplified to log (True or False):
• main functionality (now called meshes_from_gmsh) now lives in a separate file in meshlib.
• msh2vtu is still available via CLI.
• added meshes_from_gmsh to top level API.
• went through all existing msh2vtu tests, cleaned them up and added some more assertions.
• added the ability to add dynamic (.py) user-guides like in the examples
• moved and updated msh2vtu user-guide to this gallery (We should convert to other ones in the near future to benefit from this)
• small changes to avoid having some test/docs code clutter the repository.
• replaced old dev-weblinks to the weblink to the latest stable release to avoid confusion for users which don't have the master but the pip release

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behavior change was documented?